Regional coronary blood flow was measured during normal left ventricular ejection and after aortic occlusion in rats conditioned by daily swimming and sedentary controls. No significant difference was found, although the stability of the preparation was uncertain. Continued attention to this consideration may ultimately reveal a difference. Collateral coronary flow has been measured in conditioned dogs who have completed a 10-12 week treadmill running program and control dogs confined to a small room. The training program reduced the maximal exercise heart rate and increased gastrocnemius cytochrome oxidase activity by an average of 57% over paired samples from sedentary animals. Despite this evidence of conditioning, there was no significant difference in regional myocardial blood flow to normal or ischemic areas following occlusion of the left anterior descending coronary artery. Also assay samples of normal myocardium from the left circumflex coronary artery territory for myofibrillar Ca ions -MG ions ATPase, myosin Ca ions -ATPase, and myosin K ion -EDTA ATPase revealed no consistent differences between trained and sedentary dogs. Left ventricular performance and regional myocardial blood flow of sedentary and conditioned dogs were studied during control conditions, inhalation of 10% oxygen mixture, and during isoproterenol infusion. With each intervention left ventricular flow increased, but in the trained animals the increment in right ventricular flow was proportionally greater resulting in a lower LV/RV ratio. This ratio was not altered in the sedentary animal. No other difference between the two animal groups was apparent.